// POST: Transfers/Create
        // To protect from overposting attacks, please enable the specific properties you want to bind to, for 
        // more details see http://go.microsoft.com/fwlink/?LinkId=317598.
        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Create([Bind(Include = "TransferID,Employee,DateStart,DateBack,Office")] Transfer transfer)
        {
            if (ModelState.IsValid)
            {
                /// validation
                /// 
                if (transfer.DateStart > transfer.DateBack)
                {

                    вот тут нужно ругнуться
                    return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
                }

                db.Transfers.Add(transfer);
                await db.SaveChangesAsync();
                return RedirectToAction("Index");
            }

            return PartialView(transfer);
        }

IValidatableObject

public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)